In modern communications systems, frequency conversion is typically performed by combining in a non-linear device called a mixer circuit a radio frequency/intermediate frequency ("RF/IF") signal with a stabilized carrier that is produced by a local oscillator ("LO") circuit. The transistors used in oscillators are typically biased at class A for higher output power, while transistors used in mixers are biased in proximity to class B for optimum frequency conversion gain. Along with higher power consumption, class A oscillators also have small subharmonic injection locking ranges and higher AM to FM conversion, both of which cause increased phase (FM) noise degradation in the transceiver.
Previously, it was believed not to be possible to combine class A oscillators with class B mixers in one device or device pair while maintaining a high conversion efficiency. In attempting to combine these two functions, the non-linearity of the device in the oscillator circuit was typically exploited to obtain frequency conversions in the so-called "self-oscillating" mixer. Unfortunately, such attempts have failed to achieve high conversion efficiency because while the operating point is appropriate for oscillators, it is not optimum for mixers.
The locking band and phase noise degradation in a subharmonically locked oscillator are controlled by the non-linearity of the device, the device noise power and the circuit topology of the oscillator. The most important variable in a mixer, the conversion gain/loss, is also dependent upon device non-linearity, as well as the circuit topology of the mixer. Thus, the non-linearity of the device depends on the parameters of the device itself and its operating point.
Because circuit designers have been unsuccessful in combining mixers and oscillators in one device or device pair, modern frequency convertor circuits are relatively large and, as a whole, can include five or more transistors along with passive component circuitry. Similarly, the operating point for a field effect transistor ("FET") oscillator is frequently at class A, but for a FET mixer, it is at class B. Operating close to class B, the non-linearity of the FET device can be enhanced. A large non-linearity increases the subharmonic injection locking range of the oscillator, thus minimizing the phase noise degradation, and enhancing the conversion gain of the mixer. This design approach is, therefore, based on achieving stable class B operation for both the mixer and oscillator.
The necessity of large-volume monolithic microwave integrated circuits ("MMICs") for mobile communications and satellite communications has increased considerably since about 1985. Specialized subsystem circuit topologies in transmitter/receiver (T/R) arrays compatible with the MMIC fabrication must be envisioned to realize efficient large phased array antennas at millimeter wavelengths. The most useful topology is one which employs a minimal number of devices, occupies a minimum space, is small, and consumes low power, while providing high conversion efficiency.
Because information is converted up to millimeter wavelengths for transmission and down-converted to base band in the reception, LO and mixer combinations are quite frequently used for achieving this frequency translations. These oscillators and mixers are realized by use of transistors which are conventionally operated at different operating ("Q") points.
There are a number of prior art patents directed to self-oscillating mixers and mixer circuits. German Patent No. 3,813,865 discloses a self-sustaining converter with push pull mixer. The device disclosed in the German patent is intended for use with identically conductive transistors and is limited since it has an operation frequency of only up to 500 megahertz UHF, a low RF and low IF isolation point and low frequency conversion gain. Furthermore, this circuit requires a balun for asymmetric to symmetric input to output transformation and is limited to the common base configuration.
Several U.S. patents are directed to mixer circuits. For example, U.S. Pat. No. 4,593,411, Schiller discloses a diode mixer arrangement including a mixer stage connected for receiving a signal having a frequency to be converted, and an oscillator signal having one of a plurality of preselectable oscillator frequencies. The teachings of the Schiller patent are specifically incorporated herein by reference.
A similar device is taught in U.S. Pat. No. 5,175,885, Halloran et al., which discloses an improved wideband double balanced GaAs mixer including a plurality of FETs which are operative to couple a plurality of input signals to a diode ring guad in which the signals are mixed, producing output IF signals comprising multiples of the sum and difference frequency of the alternating input signals. Additionally, U.S. Pat. No. 5,003,620, Tenjin, discloses plural circuits for generating intermediate frequency signals such as UHF, VHF, and HYPER intermediate frequency signals provided in parallel. Each of the circuits includes an intermediate frequency amplifier whose output impedance varies between high and low impedances. The teachings of both the Halloran, et al. and Tenjin patents are also specifically incorporated herein by reference.
None of the aforementioned prior oscillator or mixer circuits combines the functions of oscillators and mixers in a single circuit. Thus, prior devices have been simply unable to provide both these functions in a cost-effective and efficient manner since these two functions have heretofore been implemented in separate circuits which operate at different Q points. This requires separate circuit topologies when frequency conversion is required, thereby drastically increasing hardware costs in communications systems. The art has not to date devised a solution to these problems.